Some engines may be configured to utilize a fuel that includes a mixture or blend of different fuel components. As one example, some engines can utilize E85 which includes a mixture of approximately 85% ethanol and 15% gasoline. Still other engines may be configured as a flex-fuel engine, whereby a plurality of different fuel mixtures may be used by the engine. For example, a flex-fuel engine may be configured to utilize a variety of different blends of ethanol and gasoline including up to 100% gasoline, E10 which includes approximately 10% ethanol and 90% gasoline, E85, and up to 100% ethanol. Thus, engines can utilize a variety of different fuel mixtures. Alternatively, other biofuels such as methanol may be used.
The inventors herein have recognized that the use of fuel mixtures that include ethanol or other biofuels such as methanol can result in reduced combustion quality during lower temperature conditions. The inventors have noted that ethanol has a higher temperature of vaporization than gasoline. Thus, the rate of vaporization of the mixed fuel is reduced as the relative concentration of ethanol in the fuel increases. During a start-up of the engine, such as from ambient temperature conditions which may be referred to as a cold start, the reduced vaporization of the mixed fuel due to increased ethanol concentrations may be insufficiently combusted and may result in engine misfire or stall. Thus, under these conditions, one approach has been to increase the total amount of fuel delivered to the engine in order to ensure that sufficient combustion of the fuel occurred. However, the use of additional fuel as a remedy to the reduced vaporization of the mixed fuel can result in increased levels of unburned fuel and products of combustion that are exhausted by the engine.
To address at least some of the above issues, the inventors have provided, as one example, an engine system for a vehicle, including an internal combustion engine having at least one cylinder; a fuel system configured to provide a fuel to the cylinder; an ignition system including at least a spark plug; a control system configured to vary a level of ignition energy provided to the cylinder via the spark plug in response to a composition of the fuel provided to the cylinder by the fuel system. As one example, the control system can respond to a fuel having mixtures of gasoline and alcohol (such as ethanol) in varying relative amounts. A method of operating the engine system by varying a level of ignition energy provided to the engine after a start-up is also provided, whereby the level of ignition energy can be adjusted in response to the temperature of the engine and/or the number of combustion events that have occurred since start-up. In some examples, the adjustment of ignition energy may be accompanied by an adjustment in the amount of fuel delivered to the engine for a given air charge.
In this way, combustion quality can be improved during lower engine temperature conditions regardless of the composition of fuel that is available to the engine. Additionally, by operating the ignition system to provide increased levels of ignition energy under select operating conditions, accelerated degradation of the ignition system that may result from the increased ignition energy may be reduced or minimized. Further, such an approach may also be extended to hot restarts under selected conditions.